Simple Spring integration UDP example does not respond or release client

Question

I made a simple Spring Boot project to listen for UDP transmissions and save the results. For the purposes of this question I will just have the handler print out the input.

Here is the configuration bean:

package com.wheresjim.udp.config

import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration
import org.springframework.integration.dsl.IntegrationFlow
import org.springframework.integration.ip.udp.UnicastReceivingChannelAdapter

@Configuration
class UdpConfig {

    private Integer port = 6767

    @Bean
    IntegrationFlow processUdpMessage() {
        IntegrationFlow
                .from(new UnicastReceivingChannelAdapter(port))
                .handle("udpService", "receive")
                .get()
    }
}

Here's the handler:

package com.wheresjim.udp.service

import org.springframework.messaging.Message
import org.springframework.stereotype.Component;

@Component
class UdpService {

    void receive(Message message) {
        String data = new String((byte[]) message.getPayload())
        System.out.print(data)
    }
}

Here's the client:

DatagramSocket socket = new DatagramSocket()
InetAddress address =  InetAddress.getByName('localhost')

String sendUDP(String msg, address, socket) {
    byte[] buf = msg.getBytes()
    println msg.getBytes().length
    DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 6767)
    socket.send(packet)
    packet = new DatagramPacket(buf, buf.length)
    socket.receive(packet)
    String received = new String(packet.getData(), 0, packet.getLength())
    return received
}

String testMsg = '<?xml version="1.0" encoding="UTF-8"?><TestMsg><Text>Hello World</Text></TestMsg>''


try {
    sendUDP(testMsg, address, socket)
} catch (Exception e) {
    e.printStackTrace()
} finally {
    socket.close()
}

When I send a message, it is received and displayed correctly. However there is not an acknowledgment or reply sent to the client. As such the client just hangs and I have to restart to send another message. How can I get this to send a simple reply to allow multiple messages to be sent?

Please note, the language used is Groovy

Answer 1

Thanks to the help from Artem and Gary, I was able to put together a simple solution. I added a method to my UDPService that ACKs the incoming message, and I call it when I am finished processing the input:

void ackUdp(Message message) {

    String ackMessage = ''
    DatagramPacket ackPacket = new DatagramPacket(
            ackMessage.getBytes(),
            ackMessage.length(),
            InetAddress.getByName(message.getHeaders().get('ip_address')),
            message.getHeaders().get('ip_port')
    )

    DatagramSocket socket = new DatagramSocket()

    try {
        socket.send(ackPacket)
    } catch (Exception e) {
        e.printStackTrace()
    } finally {
        socket.close()
    }
}